1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method, and more particularly relates to a system for suppressing ejection failure due to ink viscosity increasing in a nozzle of a print head.
2. Description of the Related Art
With print heads that eject ink, it is known that ejection failure such as deviations in the ejection direction and variation in ejection volumes may occur due to ink viscosity increasing inside nozzles. Decreases in ink droplet sizes and diversification in ink color materials in connection with recent improvements in print image quality have made such ejection failure occur more readily.
Since a large variety of images are printed by printers, there are cases where, depending on the print image, some nozzles are not used for comparatively long amounts of time during a printing operation. Ink viscosity increasing readily occurs in such unused nozzles, and an ejection failure occurs more readily as a result. Particularly, with large-sized inkjet printers, since it takes a comparatively long time (1 sec, for example) to cause a print head to scan from one end of a print medium to the other end, nozzle nonuse time becomes longer. Consequently, the ejection failure occurs readily with such printers.
Particularly, with multi-pass printing techniques that perform printing by a plurality of scans of a print head over the same pixel line in a scan direction, since the print data to be printed onto a single pixel line is distributed across plural scans, the frequency of ejection from a nozzle corresponding to that pixel line during a single scan decreases. For this reason, the ejection failure as discussed above occurs even more readily with multi-pass printing.
In order to prevent such ejection failure from occurring, a preliminary ejection is performed in many printers. Specifically, an ink receiving member is provided in a non-printing area of a printing apparatus, a given number of times of preliminary ejection are performed from respective nozzles of a print head into the ink receiving member at fixed intervals or at required timings and the ink inside the nozzles is refreshed. Thus, even if there exists some ink of viscosity increasing, it is discharged from the nozzles, and ink viscosity inside the nozzles can be kept normal.
In order to prevent the occurrence of the ejection failure by applying such typically performed preliminary ejection to nozzles that go unused during printing as described earlier, it is conceivable to shorten the preliminary ejection interval. There are methods for increasing the preliminary ejection frequency as one form of shortening the interval, but this incurs a derivative problem in that the overall printing throughput lowers in such cases.
Another preliminary ejection technique is known besides preliminary ejection performed to an ink receiving member as described above. This technique, called on-sheet preliminary ejection, involves ejecting ink onto a print medium on the basis of data that is unrelated to the print data (for example, see Japanese Patent Laid-Open No. 2004-025627). According to this technique, even in cases where there exist nozzles that go unused for comparatively long amounts of time depending on the print data, the on-sheet preliminary ejection can be performed during printing operation for those nozzles. Thus, it becomes possible to suppress ink viscosity increasing.
However, performing preliminary ejection onto a print medium basically means that ink dots unrelated to the image that is originally supposed to be printed will be formed among the printed matter, and lowered quality of the print image is unavoidable in some cases. Particularly, with multi-pass printing, the possibility basically increases that on-sheet preliminary ejection will be performed during each of a plurality of scans across the same pixel line of the scan direction, as described earlier. Furthermore, in the case of printing an image of comparatively low density, the ejection frequency during a single scan lowers and the ink ejection interval becomes longer, and thus the necessity of on-sheet preliminary ejection increases. For this reason, in the case of printing an image of comparatively low density with a multi-pass printing technique, the reduction in the quality of the print image due to on-sheet preliminary ejection becomes significant.